"It is because the contemporary alternatives seem so one-sided and are not more evidently solutions to the problems which Thomas faced, and partly solved, that we return to him and to the tradition of theology and philosophy in which his Summa Theologiae appears: theology as the science of the first principle and this as the total knowledge of reality in its unity." -- Wayne J. Hankey, God in Himself (Oxford University Press, 1987), p.159.

In order to answer this question clearly, we must consider the meaning of the Latin word "mos"; for thus we shall be able to discover what a "moral" virtue is. Now "mos" has a twofold meaning. For sometimes it means custom, in which sense we read (Acts 15:1): "Except you be circumcised after the manner (morem) of Moses, you cannot be saved."

Sometimes it means a natural or quasi-natural inclination to do some particular action, in which sense the word is applied to dumb animals. Thus we read (2 Maccabees 1:2) that "rushing violently upon the enemy, like lions [leonum more, i.e. as lions are in the habit of doing], they slew them": and the word is used in the same sense in Psalm 67:7, where we read: "Who maketh men of one manner [moris] to dwell in a house."

Now "moral" virtue is so called from "mos" in the sense of a natural or quasi-natural inclination to do some particular action. And the other meaning of "mos," i.e. "custom," is akin to this: because custom becomes a second nature, and produces an inclination similar to a natural one.